The main element is now defined in HTML 5.1
A Rationale / Use Case document for the addition of the
main feature to HTML is available.
From publicly reported studies  of
id values used in HTML content, the use of the strings "content" and "main" are common practice. A recent study has provided data which shows a strong correlation between the use of the aforementioned
id values as targets for 'skip links' and also a strong correlation with the use of ARIA
role=main on elements having the aforementioned
 http://triin.net/2006/06/12/CSS#figure-34, http://westciv.typepad.com/dog_or_higher/2005/11/real_world_sema.html, http://dev.opera.com/articles/view/mama-common-attributes/#id, http://lists.w3.org/Archives/Public/public-html/2012Oct/0109.html
Thanks to Mike [tm] Smith, Dave Bolter, Leonie Watson, Gez Lemon, Sarah Bourne, Henri Sivonen, James Craig, Simon Pieters, Mat Marquis, Ian Devlin, Marcos Caceres, Leif Halvard Silli and many others. Special thanks to Hixie for changing Bruce Lawson's mind.
The development of this publication is being funded by The Paciello Group. The content of this publication does not necessarily reflect the views or policies of The Paciello Group, nor does mention of trade names, commercial products, or organizations imply endorsement by The Paciello Group.